U.S. patent application number 14/825839 was filed with the patent office on 2016-12-29 for solid state wideband fourier transform infrared spectrometer.
The applicant listed for this patent is BAE SYSTEMS Information & Electronic Systems Integration Inc.. Invention is credited to Peter A. Ketteridge, Paul R. Moffitt.
Application Number | 20160377482 14/825839 |
Document ID | / |
Family ID | 57601174 |
Filed Date | 2016-12-29 |
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United States Patent
Application |
20160377482 |
Kind Code |
A1 |
Moffitt; Paul R. ; et
al. |
December 29, 2016 |
SOLID STATE WIDEBAND FOURIER TRANSFORM INFRARED SPECTROMETER
Abstract
A compact, low cost FTIR spectrometer with no moving parts
includes an interferometer having optical paths through silicon
waveguides. The optical path lengths are varied by changing the
temperature and/or carrier density of at least one of the
waveguides. In embodiments, the interferometer is a Mach-Zehnder
interferometer. Embodiments vary both optical path lengths in
opposite directions. In embodiments, a germanium or InGaAs IR
detector is grown on the same wafer as the waveguides. Embodiments
include a laser pump, such as a COT CW diode laser, and wavelength
mixer, such as an OPGaAs or OPGaP converter, for up and/or down
converting measured IR wavelengths into a range compatible with the
waveguide and detector materials. The wavelength mixer can be a
waveguide. Embodiments include a sample compartment and an IR
source such as a glowbar. In embodiments, the sample compartment
can be exposed to ambient atmosphere for analysis of gases
contained therein.
Inventors: |
Moffitt; Paul R.; (Hollis,
NH) ; Ketteridge; Peter A.; (Amherst, NH) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
BAE SYSTEMS Information & Electronic Systems Integration
Inc. |
Nashua |
NH |
US |
|
|
Family ID: |
57601174 |
Appl. No.: |
14/825839 |
Filed: |
August 13, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62079185 |
Nov 13, 2014 |
|
|
|
Current U.S.
Class: |
250/339.08 |
Current CPC
Class: |
G01J 3/108 20130101;
G01J 3/453 20130101; G01N 21/3504 20130101; G01N 2021/3595
20130101; G01J 3/4531 20130101; G01J 3/0205 20130101; G01B 9/02015
20130101; G01J 3/4532 20130101 |
International
Class: |
G01J 3/453 20060101
G01J003/453; G01B 9/02 20060101 G01B009/02; G01N 21/3504 20060101
G01N021/3504; G01J 3/02 20060101 G01J003/02; G01J 3/10 20060101
G01J003/10 |
Claims
1. A Fourier Transform Infrared ("FTIR") Spectrometer, comprising:
a controller; an optical signal input; an optical interferometer
configured to receive an FTIR input wave from said optical signal
input, said optical interferometer having at least two light paths,
each of said light paths being directed through a waveguide
comprising a waveguide material, all of said light paths being
fixed in physical length, at least one of said light paths being
variable in optical length by changing an index of refraction of
the waveguide material of the light path under control of said
controller; and an infrared detector, configured to receive and
detect an output of the optical interferometer.
2. The FTIR spectrometer of claim 1, wherein the interferometer is
a Mach-Zehnder interferometer having two light paths.
3. The FTIR spectrometer of claim 2, wherein the optical lengths of
both of the two light paths are variable in optical length under
control of said controller.
4. The FTIR spectrometer of claim 1, wherein the optical length is
variable by controlling a temperature of the waveguide material of
the at least one light path.
5. The FTIR spectrometer of claim 1, wherein the optical length is
variable by controlling a carrier concentration of the waveguide
material of the at least one light path.
6. The FTIR spectrometer of claim 1, wherein said waveguide
material is silicon.
7. The FTIR spectrometer of claim 1, wherein the waveguide is
formed on a silicon wafer, and the FTIR detector is a germanium
detector that is grown on the silicon wafer.
8. The FTIR spectrometer of claim 1, wherein the waveguide is
formed on a silicon wafer, and includes heterogeneous integration
of an indium gallium arsenide (InGaAs) detector diode as the FTIR
detector.
9. The FTIR spectrometer of claim 1, further comprising a
wavelength converter, said wavelength converter comprising a pump
laser and an optical mixing medium.
10. The FTIR spectrometer of claim 9, wherein the optical mixing
medium is OpGaAs.
11. The FTIR spectrometer of claim 9, wherein optical mixing medium
is LiNbO.sub.4 (LN) or Zinc Germanium Phosphide.
12. The FTIR spectrometer of claim 9, wherein the optical mixing
medium is included in a mixing waveguide device.
13. The FTIR spectrometer of claim 9, wherein the pump laser
comprises a COT CW diode laser.
14. The FTIR spectrometer of claim 1, further comprising a sample
compartment configured to contain an FTIR test sample and to allow
an infrared measurement wave to pass through the FTIR test sample,
said FTIR input wave being derived from said infrared measurement
wave.
15. The FTIR spectrometer of claim 14, further comprising an
onboard active FTIR light source configured to generate the
infrared measurement wave.
16. The FTIR spectrometer of claim 15, wherein the onboard active
light source is a glowbar.
17. The FTIR spectrometer of claim 14, wherein the sample
compartment can be configured to be in gas communication with a
surrounding atmosphere for analysis of the gases contained
therein.
18. A method for performing Fourier Transform Infrared ("FTIR")
spectrometry comprising the steps of: producing an infrared wave;
passing the infrared wave through a sample; mixing the infrared
wave with an incoming pump wave; producing an auxiliary wave,
wherein the auxiliary wave is produced from mixing the infrared
wave with the pump wave, wherein the auxiliary wave has a
wavelength within a usable range of a first waveguide, a second
waveguide, and a detector, said first and second waveguides being
fixed in physical length; splitting the auxiliary wave into a first
beam and a second beam; passing the first beam through the first
waveguide, wherein the first waveguide has a first optical length;
passing the second beam through the second waveguide, wherein the
second waveguide has a second optical length; changing an index of
refraction of the second waveguide, wherein changing the index of
refraction creates a difference between the second optical length
and the first optical length; and causing the first and second
beams to converge and using the detector to detect an interference
between the first and second beams.
19. The method of claim 18, wherein changing the index of
refraction of the second waveguide comprises at least one of
heating the second waveguide and changing a carrier concentration
of the second waveguide.
20. The method of claim 18, further comprising changing an index of
refraction of the first waveguide, such that one of the first
optical length and the second optical length is increased, while
the other of the first optical length and the second optical length
is decreased.
Description
RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Application No. 62/079,185, filed Nov. 13, 2014, which is herein
incorporated by reference in its entirety for all purposes.
FIELD OF THE INVENTION
[0002] The invention relates to infrared spectrometry, and more
particularly, to Fourier transform infrared spectrometers.
BACKGROUND OF THE INVENTION
[0003] Fourier transform infrared ("FTIR") spectrometers have been
in use for decades for routine chemical analysis. The FTIR concept
is built on the use of an interferometer that can scan over many
null and maximum points. Typically, FTIR spectrometers have used
mechanical interferometers that are based on the Michelson design.
These mechanical FTIR systems have demonstrated excellent
performance over a wide wavelength range. In addition, some
implementations of the Michelson approach have been made smaller
and more rugged over time through careful design. However, a high
degree of mechanical precision and cost has been required to
realize such improved implementations. Also, even for the more
rugged configurations, the mechanical nature of the Michelson
interferometer design renders it intrinsically susceptible to
misalignment and/or damage.
[0004] What is needed, therefore, is an FTIR spectrometer that can
be manufactured at a reduced price and size, and that includes no
moving parts.
SUMMARY OF THE INVENTION
[0005] A novel FTIR spectrometer that includes no moving parts and
is less expensive to manufacture and smaller in size than
conventional FTIR spectrometers based on the Michelson design
includes a Mach-Zehnder interferometer that is realized in silicon.
The optical path lengths of the two waveguide arms of the
Mach-Zehnder interferometer are varied by changing the index of
refraction of the waveguide material. In some embodiments, this is
done by heating the waveguides, while in other embodiments this is
accomplished by changing the carrier concentration of the Si in the
waveguides. This latter approach is achieved by configuring each
arm of the interferometer as either a "p-n" or "p-i-n" diode, and
by changing the electrical bias conditions of the diodes. In
embodiments, the optical path lengths of both of the arms are
driven in opposite directions, that is, one arm is made optically
longer, while the other arm is made optically shorter.
[0006] The use of silicon for the waveguide material significantly
reduces the cost of manufacturing the interferometer, and in
various embodiments a germanium detector is grown on the wafer,
thus simplifying the packaging and reducing cost still further.
Other embodiments include heterogeneous integration of an indium
gallium arsenide ("InGaAs") detector diode.
[0007] The use of silicon waveguides and germanium or InGaAs
detectors in various embodiments limits the operating wavelength
range of the interferometer to between 1.2 .mu.m (onset of
absorption of the silicon waveguide) and 1.6 .mu.m (absorption edge
of the germanium detector) or 1.8 .mu.m (absorption edge of an
InGaAs detector). However, this small working range can be
effectively expanded by up-converting longer wavelengths and
down-converting shorter wavelengths so that they fall within the
usable range of the interferometer/detector combination. For
example, a pumping laser and a frequency mixer such as an oriented
patterned gallium arsenide ("OPGaAs") or oriented patterned gallium
phosphide ("OPGaP") mixer can be used to perform the up-conversion
or down-conversion. Similar embodiments include frequency sum
and/or difference mixers based on any of a variety of non-linear
materials and methods, including periodically poled lithium niobate
and zinc germanium phosphide.
[0008] In various embodiments, the pump laser is tunable over the
band of interest. In some embodiments, a proper choice of laser
materials allows the complete IR band to be measured, while in
other embodiments the pump laser is configured to measure only a
limited wavelength band where absorption lines of interest are
known to exist.
[0009] In exemplary embodiments of the present invention, the
spectrometer is used in a standoff configuration, wherein the
sample to be measured is located at some distance from the
spectrometer. Other embodiments take advantage of the small size of
the invention to eliminate the 1/R.sup.2 loss in sensitivity that
is typical of standoff sampling systems by placing the sample in a
sample space provided within the spectrometer, so that the
interferometer is very close to the sample being measured. The
spectrometer sensitivity can be greatly increased for some of these
embodiments through the use of an onboard active light source, such
as a glow bar. In embodiments, the sample space can be exposed to
the surrounding air to allow detection and analysis of any chemical
that may be present in the ambient atmosphere.
[0010] Materials such as LiNbO.sub.4 ("LN"), Zinc Germanium
Phosphide ("ZGP"), and many other materials, some of which are well
established and others of which are still under development,
possess both transparency and high optical polarizability, and can
be used in various embodiments in place of OpGaAs as an
up-converting and/or down-converting medium.
[0011] The recent perfection of parallel technology in both
non-linear poled materials such as OpGaAs and Si-photonics
platforms is a key enabler of the present invention. In some
embodiments, waveguide devices constructed of these new converter
materials allow very small, commercially available
cyclooctatetraene ("COT") CW laser diodes to generate the baseband
spectral inputs ("BBSI") from approximately 1.2 to 1.6 microns that
are required by these Si/SiO.sub.2 filter devices. The CW outputs
of these COT laser diodes, typically with output powers as low as
1-10 milliwatts, experience long interaction lengths in the guided
modes of the waveguides, which efficiently converts, for example,
the long-wave, information-rich spectral telltale signatures in the
signal to the BBSI wavelength range. The relative spectral
locations, widths, and intensities are thereby mapped into the
converted wavelengths.
[0012] This waveguide conversion approach overcomes the natural
tendency of tightly focused optical drive or pump beams to diverge,
and thereby maintains the beam intensities that are required for
conversion. This enables the use of simple CW pumps, which provide
electrical and optical simplicity, because it eliminates the need
for short-pulse pump lasers, which typically require complex and
inefficient electro-optical components and accompanying high flux
and peak power levels that are known to place stress on other
material in the beam path.
[0013] One general aspect of the present invention is a Fourier
Transform Infrared ("FTIR") Spectrometer which includes a
controller, an optical signal input, an optical interferometer
configured to receive an FTIR input wave from said optical signal
input, said optical interferometer having at least two light paths,
each of said light paths being directed through a waveguide
comprising a waveguide material, all of said light paths being
fixed in physical length, at least one of said light paths being
variable in optical length by changing an index of refraction of
the waveguide material of the light path under control of said
controller, and an infrared detector, configured to receive and
detect an output of the optical interferometer.
[0014] In embodiment, the interferometer is a Mach-Zehnder
interferometer having two light paths. And in some of these
embodiments, the optical lengths of both of the two light paths are
variable in optical length under control of said controller.
[0015] In any of the above embodiments, the optical length can be
variable by controlling a temperature of the waveguide material of
the at least one light path. In any of the above embodiments, the
optical length can be variable by controlling a carrier
concentration of the waveguide material of the at least one light
path.
[0016] In any of the embodiments listed above, said waveguide
material can be silicon. In any of the preceding embodiments, the
waveguide can be formed on a silicon wafer, and the FTIR detector
can be a germanium detector that is grown on the silicon wafer.
[0017] In any of the embodiments mentioned above, the waveguide can
be formed on a silicon wafer, and can include heterogeneous
integration of an indium gallium arsenide (InGaAs) detector diode
as the FTIR detector.
[0018] Any of the above embodiments can further include a
wavelength converter, said wavelength converter comprising a pump
laser and an optical mixing medium. In some of these embodiments,
the optical mixing medium is OpGaAs. In other of these embodiments,
optical mixing medium is LiNbO4 (LN) or Zinc Germanium Phosphide.
And for any of the preceding embodiments, the optical mixing medium
can be included in a mixing waveguide device, and/or the pump laser
can comprise a COT CW diode laser.
[0019] Any of the above embodiments can further include a sample
compartment configured to contain an FTIR test sample and to allow
an infrared measurement wave to pass through the FTIR test sample,
said FTIR input wave being derived from said infrared measurement
wave. Embodiments further include an onboard active FTIR light
source configured to generate the infrared measurement wave, and in
some of these embodiments the onboard active light source is a
glowbar. In various of these embodiments the sample compartment can
be configured to be in gas communication with a surrounding
atmosphere for analysis of the gases contained therein.
[0020] Another general aspect of the present invention is a method
for performing Fourier Transform Infrared ("FTIR") spectrometry.
The method includes: [0021] a. producing an infrared wave, [0022]
b. passing the infrared wave through a sample, [0023] c. mixing the
infrared wave with an incoming pump wave, [0024] d. producing an
auxiliary wave, wherein the auxiliary wave is produced from mixing
the infrared wave with the pump wave and the auxiliary wave has a
wavelength within a usable range of a first waveguide, a second
waveguide, and a detector, said first and second waveguides being
fixed in physical length, [0025] e. splitting the auxiliary wave
into a first beam and a second beam, passing the first beam through
the first waveguide, wherein the first waveguide has a first
optical length, [0026] f. passing the second beam through the
second waveguide, wherein the second waveguide has a second optical
length, [0027] g. changing an index of refraction of the second
waveguide, wherein changing the index of refraction creates a
difference between the second optical length and the first optical
length, and [0028] h. causing the first and second beams to
converge and using the detector to detect an interference between
the first and second beams.
[0029] In embodiments, changing the index of refraction of the
second waveguide comprises at least one of heating the second
waveguide and changing a carrier concentration of the second
waveguide.
[0030] And some embodiments further include changing an index of
refraction of the first waveguide, such that one of the first
optical length and the second optical length is increased, while
the other of the first optical length and the second optical length
is decreased.
[0031] The features and advantages described herein are not
all-inclusive and, in particular, many additional features and
advantages will be apparent to one of ordinary skill in the art in
view of the drawings, specification, and claims. Moreover, it
should be noted that the language used in the specification has
been principally selected for readability and instructional
purposes, and not to limit the scope of the inventive subject
matter.
BRIEF DESCRIPTION OF THE DRAWINGS
[0032] FIG. 1 is a block diagram illustrating an embodiment of the
FTIR spectrometer of the present invention;
[0033] FIG. 2A is a simplified diagram illustrating the signal,
pump, and converted waves that are present in the wavelength
converter of FIG. 1;
[0034] FIG. 2B illustrates the momentum relationship and photon
mixing between the signal, pump, and converted waves of FIG.
2A;
[0035] FIG. 2C is a graph that illustrates the relationship between
the photon flux densities of the signal and converted waves, under
an assumption that the pump photon-flux density is constant;
and
[0036] FIG. 3 is a graph that illustrates the difference frequency
mixing pump wavelength variance that is required to convert a
signal in the mid-infrared input spectrum and the long wave
infra-red input spectrum into a wavelength band that is compatible
with the silicon waveguides and germanium detector used in
embodiments of the present invention.
DETAILED DESCRIPTION
[0037] With reference to FIG. 1, a novel FTIR spectrometer 100 that
is less expensive to manufacture and smaller in size than
conventional FTIR spectrometers based on the Michelson design
includes a Mach-Zehnder interferometer 102 that is realized in
silicon and includes no moving parts. The optical path lengths of
the two waveguide arms 104a, 104b of the interferometer 102 are
varied under control of interface electronics 118 by changing the
index of refraction of the waveguide material. In some embodiments,
this is done by heating the waveguides, while in other embodiments
this is accomplished by changing the carrier concentration of the
Si in the waveguides. This latter approach is achieved by
configuring each arm 104 as either a "p-n" or "p-i-n" diode and by
changing the electrical bias conditions of the diodes. In
embodiments, the optical path lengths of both of the arms 104 are
driven in opposite directions; that is, one arm 104a is made
optically longer, while the other arm 104b is made optically
shorter.
[0038] The use of silicon for the waveguide material significantly
reduces the cost of manufacturing the interferometer 102. In
embodiments, a typical 6'' silicon wafer can yield several hundred
such devices. In various embodiments, a germanium detector 106 is
grown on the wafer, thus simplifying the packaging and reducing
cost still further. Other embodiments include heterogeneous
integration of an indium gallium arsenide ("InGaAs") detector
diode.
[0039] In the embodiment of FIG. 1, the spectrometer 100 further
includes a control system 120 that drives the interface electronics
and analyzes the signals received from the germanium detector
106.
[0040] The use of silicon waveguides 104 and germanium detectors
106 or InGaAs detectors in various embodiments limits the operating
wavelength range of the interferometer 102 to between 1.2 .mu.m
(onset of absorption of the silicon waveguide) and 1.6 .mu.m
(absorption edge of the germanium detector) or 1.8 .mu.m
(absorption edge of an InGaAs detector). However, this small
working range can be expanded by up-converting longer wavelengths
and down-converting shorter wavelengths so that they fall within
the useful range of the interferometer/detector combination. In the
embodiment of FIG. 1, this is accomplished by a pumping laser 108
and an oriented patterned gallium arsenide ("OPGaAs") frequency
difference mixer 110. Similar embodiments include frequency sum
and/or difference mixers based on any of a variety of non-linear
materials and methods, including periodically poled lithium
niobate, zinc germanium phosphide, and oriented patterned gallium
phosphide ("OPGaP").
[0041] In the embodiment of FIG. 1, an optical interface block 112
couples the energy from the wavelength converter 110 into the very
small Si waveguide 104. In some embodiments, this is accomplished
by direct focusing of the energy onto the end of the waveguide 104.
In other embodiments, a grating is used to couple the energy from
the converter 110 at an incident angle to the end of the waveguide
104. In still other embodiments, a fiber optic lead is used to
guide the energy to the waveguide 104 through the use of tapered
fibers or ball lenses.
[0042] In various embodiments, the laser 108 is configured to be
tuned to cover the band of interest. In some embodiments, a proper
choice of laser materials allows the complete IR band to be
measured, while in other embodiments the laser is configured to
measure only a limited wavelength band where absorption lines of
interest are known to exist.
[0043] In exemplary embodiments of the present invention, the
spectrometer is used in a standoff configuration, wherein the
sample to be measured is located at some distance from the
spectrometer. With reference to FIG. 1, other embodiments take
advantage of the small size of the invention to eliminate the
1/R.sup.2 loss in sensitivity that is typical of standoff sampling
systems by placing the sample in a sample space 114 provided within
the spectrometer 100, so that the interferometer 102 is very close
to the sample being measured 114. The spectrometer sensitivity can
be greatly increased for some of these embodiments through the use
of an onboard active light source 116, such as a glow bar. In
embodiments, the sample space 114 can be exposed to the surrounding
air to allow detection and analysis of any chemical that may be
present in the ambient atmosphere.
[0044] Materials such as LiNbO.sub.4 (LN), Zinc Germanium Phosphide
(ZGP), and many other materials, some of which are well established
and others of which are still under development, possess both
transparency and high optical polarizability and can be used in
various embodiments in place of OpGaAs 110 as an up-converting
and/or down-converting medium. Through non-linear optical mixing, a
desired IR band can thereby be translated so that it falls with the
usable range of the Si waveguides 104 and the integrated Ge
detectors 106, thereby enabling them to detect a broader range of
compounds at their fundamental ID wavelengths, where the
distinguishing spectral features are stronger.
[0045] As noted above, the incoming signal can be mixed with an
on-board 108 laser using either Difference Frequency Mixing
("DFM"), or Sum Frequency Mixing ("SFM"). Either technique is
usable, but for simplicity only DFM is described herein. One of
skill in the art will readily perceive how SFM can be applied in a
similar manner.
[0046] The recent perfection of parallel technology in both
non-linear poled materials such as OpGaAs and Si-photonics
platforms is a key enabler of the present invention. With reference
to FIG. 2A, the Difference Frequency Mixer (DFM) approach uses
three-wave mixing in a nonlinear material to provide optical gain.
The process is governed by three coupled energy exchange equations
with the waves identified as follows:
[0047] The first wave, .omega.1, is the input signal, and is
incident on the crystal 110 with a small input intensity I1(0). A
second wave, .omega.3, the pump, is an intense wave that provides
power to the mixer 110. The newly generated converted wave,
.omega.2, is an auxiliary wave created by this interaction process.
In the DFM case, .omega.2 represents the up-converted frequency
suitable for injection into the silicon waveguide 104. Energy
conversion dictates that
.omega.2=.omega.3-.omega.1 (1).
[0048] The coherent growth of the preferred optical frequencies
along the axis of propagation is assured by the matching of the
different frequency fields with dispersion invoked momentum
matching, according to
k3=k1+k2 (2)
Which can be written
h.omega.3=h.omega.1+h.omega.2 (3)
[0049] as illustrated in FIG. 2B. DFM has the additional merit of
providing gain as long as the applied pump intensity is
sufficiently high. Small signal conversion efficiency in the
mid-infrared wavelength range has been shown to reach 175% v with
150 mW of laser diode pump energy.
[0050] The weak input beams .omega.1 are collected and guided into
the converter material volume 110 and mixed with the output of the
driving pump laser 108. The high intensity of the pump beam
.omega.3, in conjunction with the input beam .omega.1, deforms the
charge clouds surrounding the molecules of the converter material.
The beat of these two beams .omega.1, .omega.3 drives the high
order wave propagation, such that new electric fields are
generated. Due to the natural or engineered dispersion of the
converter material, as illustrated in FIG. 2C, a narrow portion of
these harmonic polarization fields matches the phase speeds .phi.1,
.phi.2 of the two original beams. This causes both the signal and
converted waves to grow as individual pump photons "split" into a
modified signal beam and a newly generated, converted beam.
[0051] FIG. 3 depicts the range of mid wave and long wave infrared
inputs (four dashed curves) and their resulting converted
wavelengths (vertical axis) as a function of the pump wavelength,
(horizontal axis) for an exemplary embodiment. It can be seen from
the figure that the two bands representing the signature-rich
spectral bands which overlap with good atmospheric transmission,
namely the 3-5 um and the 8-12 um bands, are up-converted by the
OpGaAs converter 110 into the vertical limits of the cross-hatched
box region in the figure, which is demarked on the vertical axis as
1.2 to 1.6 um. The straight vertical lines on the left and right
sides of the box define the pump laser central wavelengths required
to achieve this converted output.
[0052] As can be seen from the figure, by varying the central
wavelength of the pump laser 108, the two aforementioned spectral
inputs can be converted into the detector's spectral response
window by varying the pump wavelength from 1.04 to 1.09 um. In some
embodiments, cyclooctatetraene ("COT") CW laser diodes are used to
meet these requirements, while in other embodiments a broadband
laser source such as a CW laser diode pumped broadband laser having
this range of spectral output is used.
[0053] Additionally, in some embodiments waveguide devices
constructed of these new converter materials allow very small,
commercially available COT laser diodes to generate the baseband
spectral inputs ("BBSI") from approximately 1.2 to 1.6 microns that
are required by these Si/SiO.sub.2 filter devices. The CW outputs
of these COT laser diodes, typically with output powers as low as
1-10 milliwatts, experience long interaction lengths in the guided
modes of the waveguides, which efficiently converts, for example,
the long-wave, information-rich spectral telltale signatures in the
signal to the BBSI wavelength range. The relative spectral
locations, widths, and intensities are thereby mapped into the
converted wavelengths.
[0054] This waveguide conversion approach overcomes the natural
tendency of tightly focused optical drive or pump beams to diverge,
and thereby maintains the beam intensities that are required for
conversion. This enables the use of simple CW pumps, which provide
electrical and optical simplicity, because it eliminates the need
for short-pulse pump lasers, which typically require complex and
inefficient electro-optical components and accompanying high flux
and peak power levels that are known to place stress on other
material in the beam path.
[0055] The foregoing description of the embodiments of the
invention has been presented for the purposes of illustration and
description. Each and every page of this submission, and all
contents thereon, however characterized, identified, or numbered,
is considered a substantive part of this application for all
purposes, irrespective of form or placement within the
application.
[0056] This specification is not intended to be exhaustive.
Although the present application is shown in a limited number of
forms, the scope of the invention is not limited to just these
forms, but is amenable to various changes and modifications without
departing from the spirit thereof. One or ordinary skill in the art
should appreciate after learning the teachings related to the
claimed subject matter contained in the foregoing description that
many modifications and variations are possible in light of this
disclosure. Accordingly, the claimed subject matter includes any
combination of the above-described elements in all possible
variations thereof, unless otherwise indicated herein or otherwise
clearly contradicted by context. In particular, the limitations
presented in dependent claims below can be combined with their
corresponding independent claims in any number and in any order
without departing from the scope of this disclosure, unless the
dependent claims are logically incompatible with each other.
* * * * *